Add a matching SDDM login screen theme

What do you need?

While we never really see the SDDM login screen due to autologin, I do think it'd be nice to have one that matches the general aesthetic of the Hyprlock screen for instances where it may be seen (eg. Hyprland crashes) or if you're a user who uses multiple user accounts and need to switch.

Most login screens I've come across are massive packs of options OR are just overly complex. I think it's important that we maintain a simple and consistent aesthetic, and preferably also update the BG that's shown on theme change as well.

Current Hyprlock for Reference